A satellite communication system for communications between a central hub station and remote earth stations equipped with so-called very small aperture terminals (VSATs) includes a hub station including an uplink, at least one geosynchronous communication satellite having a predetermined earth "footprint", and an unlimited number of VSATs for receiving transmissions from an associated communication satellite. Until the early 1980's most satellite broadcasters used C-band frequencies. Now communication satellites that broadcast radio, television, computer, and other data messages in the Ku-band frequency range are commonplace. The 10.95 to 14.50 GHz band (Ku-band) of the electromagnetic spectrum has been allocated solely for satellite transmissions. The higher frequency Ka-band (17.7 to 21.2 GHz) is being eyed by potential users for future satellite communications. One such system presently in operation using the Ku-band frequency range is the Scientific-Atlanta SkylinX.25 VSAT network which consists of a master earth station or hub, and geographically distributed VSATs which communicate therewith.
The FCC requires that: (1) the satellite EIRP not exceed 6 dBW per 4 kilohertz of bandwidth on the downlink; and that (2) the power spectral density into the feed of an earth station antenna not exceed -14 dBW per 4 kilohertz of bandwidth on the uplink. For a given modulation technique, these standards place an upper limit on the amount of fade caused by weather conditions, including rain, which can be tolerated on both the uplink and downlink.
As microwave communication frequencies increase, weather conditions become more likely to adversely affect reception and signal-to-noise ratio. Multipath fading results when components of a direct signal and reflections interact to reduce or increase the net received signal amplitude. The depth of fade can fluctuate widely and may cause a complete failure of transmission in one or more channels for short periods of time. As raindrop size becomes an appreciable fraction of the transmitted wavelength, variations in attenuation due to absorption or scattering increases. Therefore, it is desirable at microwave frequency levels to provide a means to insure a good signal to noise ratio at the receiver and minimize the effects of fading.
To provide a more economical VSAT system, the overall cost of the antenna used with the remote stations can be reduced in the areas of production, installation and handling. This can be accomplished by using smaller antennas which can be shipped by less expensive means and installed/setup by fewer people in a shorter time. With a smaller antenna at the remote site, the received signal-to-noise ratio (SNR) on the outbound link from the HUB station to the remote station is now diminished to an unacceptably low level. Furthermore, the smaller antenna cannot provide an adequate amount of uplink output power on the return link from the remote station to the HUB station. To increase the SNR at the remote station, the downlink power from the satellite must be increased. This can be done by increasing the uplink power from the HUB and, at the same time, reducing the energy density to remain within the FCC energy density limit at the satellite output. Energy dispersal can be used to spread the signal energy over a wider bandwidth and thereby maintain the energy density to within the FCC limit. Similarly, energy dispersal can be used on the return link to increase the power into the antenna feed while maintaining the FCC feed energy density limit.
Spreading techniques known in the art such as frequency hopping and direct sequence spreading facilitate energy dispersal. Many communications systems employ spread spectrum techniques for a variety of applications. According to conventional spread spectrum systems, message information is time and/or frequency encoded with a pseudo-noise (PN) sequence to provide a transmission signal spread over a wide bandwidth or frequency spectrum relative to the message or information bandwidth. The transmission signal passes through a selected wide band communication channel to a receiver which acquires and tracks the transmission signal timing and thereafter recovers the encoded message.
Spread spectrum systems typically incorporate a pseudo-random noise (PN) generator at the transmitter for generating, for example, a phase modulated spread spectrum signal. The receiver employs a corresponding PN generator synchronized to the transmitter PN generator for coherent detection of the message signal. The PN generators at the transmitter and receiver provide a low-level transmission signal which can be correlated with an internally generated receiver pseudo-random noise signal to improve signal to noise ratio and to increase system reliability. Once an incoming spread spectrum signal has been detected and identified, various techniques may be employed for recovering the message data. One such system is described by U.S. Pat. No. 4,977,578 to Ishigaki et al. U.S. Pat. No. B1 Re 32,905 to Baran, a reexamination of the reissue of U.S. Pat. No. 4,455,651, discloses another exemplary system, particularly directed to spread spectrum communication in a satellite communications system.
In microwave ground communications, frequency diversity over the same communications path is used where identical information is transmitted via an 11 GHz band link and a 6 GHz band link. With the same information transmitted over independent fading channels, the information quality is improved. However, in such an arrangement, dual facilities (transmitters, waveguide runs, antennas, and receivers) are required resulting in increased equipment expense.
There remains a need in the art to provide high weather availability of the communication links and moreover, to permit the utilization of smaller sized antennas of aperture sizes of around one meter. Further, the FCC limits imposed on satellite communications make it desirable to more efficiently utilize the bandwidth of the transmitted signal energy.